1. Field of the Invention
The present invention relates to a wire electric discharge machine, and particularly to a wire electric discharge machine that operates with a constant inter-pole distance.
2. Description of the Related Art
A wire electric discharge machine is an apparatus that machines a workpiece by producing electric discharge between poles of a wire electrode and the workpiece. In the wire electric discharge machining, the machining is performed with maintaining the inter-pole distance within a very small value appropriate for developing electric discharge, and it is necessary to keep the inter-pole distance constant for a precise machining result.
To keep the inter-pole distance constant, the inter-pole distance may be detected as a control variable, and an operation variable may be adjusted by feedback control, such as proportional control, in such a way that the control variable approaches a predetermined target value. The operation variable is typically a speed at which the wire electrode and the workpiece are moved relative to each other during the machining (machining feed speed).
In the wire electric discharge machining, increasing the machining feed speed causes the inter-pole distance to decrease, whereas decreasing the machining feed speed causes the inter-pole distance to increase. Therefore, it is necessary to increase the machining feed speed when the inter-pole distance is greater than the target value, or to decrease the machining feed speed when the inter-pole distance is smaller than the target value. Thus the inter-pole distance can be approach the target value.
Since it is difficult to directly detect the inter-pole distance, i.e. the control variable, during electric discharge machining, an indirectly detected value of the inter-pole distance is typically used as the control variable. That is, a physical quantity that correlates with the inter-pole distance is detected, and the detected value is used as a substitute for the control variable.
In the wire electric discharge machining, an electric discharge delay period is present after applied the voltage the poles and then the electric discharge occurs, and the greater the inter-pole distance, the longer the period. The inter-pole voltage is maintained at a high value during the electric discharge delay period and it will be reduced to a low value referred to as an arc voltage after the electric discharge occurs. Thereafter, a series of cycles is repeated with interposing a predetermined pause duration therebetween. Therefore, the greater the inter-pole distance, the longer the electric discharge delay period, and hence the longer the period for which the inter-pole voltage is high. As an actual control variable, for example, an average electric discharge delay period or an average inter-pole voltage may also be used.
Japanese Patent Laid-Open No. 2012-192520 discloses a technology for identifying short-circuit pulses and changing the machining feed speed on the basis of an average number of successive short-circuit pulses.
A machining liquid is presented between the poles, and the density of electrically conductive machining waste suspended in the machining liquid between the poles varies as the machining advances. For example, when the density of the machining waste increases, the proportion of the electric discharge produced through the machining waste increases so that the electric discharge delay period shortens in spite of the inter-pole distance remain unchanged. Thus the inter-pole distance cannot be expressed through the electric discharge delay. When an attempt is so made that the control variable is constant in this state, the machining feed speed decreases and the inter-pole distance undesirably increases. That is, it is impossible to make the inter-pole distance to be constant, so that undesirable imprecise machining is resulted. Further, in Japanese Patent Laid-Open No. 2012-192520, the average number of continuous short-circuit pulses itself is used as the control variable, and an object thereof is the improvement of the straightness precision. The average number of continuous short-circuit pulses does not correlate with the inter-pole distance.